The present invention relates to a spin coating apparatus (hereinafter referred to as a spinner) for manufacturing an electronic device such as an integrated circuit (IC) device. Particularly, it relates to a spinner suitable for forming a photoresist layer having an acceptable uniform thickness over a substrate with a non-circular outer shape.
Recently, as miniaturization and an increase in the packing density of semiconductor integrated circuit devices have been promoted, the pattern of circuits formed on a semiconductor substrate or the pattern of the associated mask, has become extremely fine such as of a submicron order. On the other hand, the size of the substrate has been enlarged, such as a wafer of eight inches in diameter, for example, to achieve cost reduction in the mass production of the associated devices. As a result, fine and precise pattern formation over the surface of a substrate having a substantially large area is required in the field. There is similar requirement in the other fields such as manufacturing of liquid crystal display devices and electro-luminescence (EL) devices. In order to meet this requirement, a great deal of effort for developing production technology to realize such fine pattern formation in a large area has been made in the field.
A technology referred to as photolithography is widely used at present, wherein patterning of circuits and photomasks is performed with a photo-etching technology. In current photolithography, a film of photo-etch resist resin (hereinafter referred to as photoresist) is widely used. In a fabrication step of a semiconductor device or other products, photoresist is currently used as a material of a patterning film coated on a substrate such as a semiconductor substrate or a reticule substrate.
There are various methods for coating photoresist film over a substrate, such as spin coating, spray coating, dip coating and roll coating. As the pattern becomes finer, the thickness of the photoresist film must be reduced in line with the width of a line contained in the pattern. For coating a thin photoresist film on a substrate having a large area, the spin coating is most suitable to provide the film with substantially uniform thickness throughout the entire area of the film. For spin coating of a photoresist film on a substrate, a spinner is used.
FIG. 1 is a schematic perspective view of a spinner of the prior art, illustrating the structure thereof. The spinner 11 comprises a rapidly rotatable turntable 13 on which a substrate 15 to be coated is held thereon using a vacuum chucking means. The vacuum is made by an evacuating device (not shown) and applied to the substrate 15 through suction holes 14. The turntable 13 is surrounded by a spin cup 12 coaxially disposed with respect to the rotating axis 0--0 of the turntable 13. Liquid photoresist is supplied from a photoresist source (not shown) and dropped on the center portion of the rotating turntable 13 through a nozzle (not shown). The dropped liquid photoresist spreads outwardly over the surface of the substrate 15 in rotation by centrifugal forces caused by rotation, and any excess supply of the photoresist is hurled off the turntable 13, and received by the spin cup 12 and drained downwardly. The thickness of thus formed photoresist film over the substrate 15 is adjusted by balancing the viscosity of the liquid photoresist and the centrifugal force caused by the rotation of the turntable 13. In the prior art spinners, in general the turntable employed has a disk having a flat top surface with no protrusion to avoid disturbing the spreading flow of the liquid photoresist during spin operation.
When a circular substrate such as a circular semiconductor wafer is coated with a photoresist film using the prior art spinner 11, the thickness of the coated film is favorably uniform. The thickness of a photoresist film coated on a non-circular substrate, such as a rectangular substrate 15 as shown in FIG. 1, however, is rather non-uniform, and further, rippled portions 18 are caused in the peripheral area of the substrate 15. This is considered to be caused by turbulent air flow adjacent to the top surface of the spinning substrate.
Generally, air contacting with the surface of a rotating body is driven to cause an air flow which runs in the same rotating direction as that of the body. The air flow is caused by the friction between the surface of the rotating body and air contacting with the surface.
The side wall of a spinning substrate also causes an air flow. When the substrate is circular, the air flow along the side wall always runs in parallel with the side wall of the substrate, resulting in a laminated flow which does not disturb the air flow adjacent to the top surface of the circular substrate. Hence, the liquid photoresist spreading over the surface of the substrate is subject to aerodynamic forces substantially being symmetrical around the rotating axis 0--0. This is favorable to make the photoresist film have a substantially uniform thickness.
When a rectangular substrate such as the substrate 15 of FIG. 1 is spun, the side walls of the rectangular substrate cause air flows with various directions and speeds, generating a turbulent flow which disturbs the air flow adjacent to the top surface of the substrate, particularly of the peripheral portion of the substrate. It is considered that the ripple marks 18 of FIG. 1 are caused by the above-described turbulent air flow exerting undesirable aerodynamic forces to the liquid photoresist film. It should be noted that the intensity of the aerodynamic forces depends on the difference between the surface speed of the film and that of the air flow adjacent to the film, namely the speed of the air flow relative to the film. As the relative speed of the air flow adjacent to a liquid photo-resist film is decreased, aerodynamic forces caused by the air flow and exerted on the film, are reduced.
In order to overcome the above-described problem with respect to the spin coating of a photoresist film over a non-circular substrate, a spinner is disclosed by Wada, in an Unexamined Japanese Patent Application, No. 55-02720, published on Jan. 10, 1980. The spinner of Wada has a modified turntable which has a portion recessed from the major top surface of the turntable. The depth and outer shape of the recessed portion are selected to coincide with those of a non-circular substrate to be processed. The substrate is loaded in the recessed portion with the result that the major top surface of the turntable and the top surface of the substrate are substantially in the same plane. This configuration serves to prevent the above-described turbulent air flow which would be caused by the side walls of the substrate. However, there is a disadvantage that at the edge portion of the substrate, the coated photoresist film tends to be damaged when the substrate is unloaded from the turntable.
Another spinner is disclosed by Yamamoto in Unexamined Japanese Patent Application No. 60-143872, published on July 30, 1985. The spinner of Yamamoto has a rotary turntable and rotary vanes rotating synchronously and coaxially with the turntable for generating an air stream which is directed downward at the outside of the turntable. The vanes are intended to change the direction of air flow caused by the rotation of the turntable. However, the generated air flow does not appear to improve the uniformity of the photoresist film.